Equipment for playing music

Mar 17 00:26 UTC 2002

Discuss the hardware aspects of playing music - tape decks, receivers,
CD-players, boomboxes, etc.

response 2 of 65:

Mar 17 00:39 UTC 2002

Jim asks what happens if you put 3 ohm speakers on a boombox that calls for
4 ohm speakers, will it damage either the speakers or the boombox if you don't
play music loudly?  He is proud to report that he is halfway through replacing
the foam surrounds of a couple of AR woofers with foam surrounds that he
carefully removed from some passive radiators (they look like speakers but
have no magnet or coil).  Today he glued (with carpenter's glue) the foam to
the cones, and tomorrow when it is dry he will glue foam to metal frame.
www.simplyspeakers.com sells new foam surrounds for $27/pair and was kind
enough to tell him old ones could be reused, and emailed instructions.  Remove
the old foam with sharp knife and alcohol.  He also glued the loose copper
coil of another speaker back into its cardboard tube, with superglue, after
carefully slicing off the dust cap (which he will glue back on tomorrow). 
It probably came loose when someone played very loud music.  THe matching
speaker to the pair had a broken wire which probably happened after the coil
came unglued.  Perhaps the glue just dried out first.

The neighbor gave him the superglue, along with a 10" woofer that has a
disintegrating cone.  Anyone have any dead 10" speakers to give away?

response 4 of 65:

Mar 18 21:12 UTC 2002

The MPX filter on tape decks is designed to get 
rid of the 19kHz pilot signal ("multiplex carrier"?) 
which is part of the FM radio signal.  That signal can trick the 
Dolby noise-reduction circuitry into misbehaving, and it may cause some
other effects; I've just noticed that in my recent taping of FM radio,
I get a lot of funny effects unless the MPX filter is on.

Someone else needs to explain the theory of how FM Stereo works, 
and how it was desgined to be compatible with monaural FM.
 
MPX filter should theoretically be left off for tapes from other sources
such as records and CDs, though at our age, and with the presumed quality 
of tape decks you are using, it probably wouldn't be a problem to
leave the 19kHz filter on all the time.   Experiment!

response 6 of 65:

Mar 18 21:58 UTC 2002

Thanks Ken.  We only listen to FM radio in mono because of the hiss in stereo
when listening to any but strong local stations (none of which we listen to)
so should be safe recording without the MPX filter.
The Beat Cut A B C is for recording from AM if there is a whistle - try out
all three until one works.

We have several timers of the sort where you push little square pins/knobs
to the inside or outside of the timer to set it to go on or off every half
hour which we may experiment with.  Our friend likes to record a particular
radio show and keeps buying VCRs to do it with because they are timed.
These are sold for lamp timers but they are also nice for turning an electric
mattress pad on or off.

The foam surround project and the glue the coil in the tube project were both
complete successes and we are now trying to determine which two of four
woofers sound better together.

The instructions that came with one tape deck had a picture of what looked
like a stereo component, labelled timer, which looked expensive.  I suppose
if we don't mind being within 10 minutes the cheap sort will also work.

There is a little picture of a clock or timer next to the play and record
switches on one tape deck.  I guess you can also set it to work as an alarm
clock but there seems to be no sleep or snooze switch.

response 8 of 65:

Mar 19 06:47 UTC 2002

Those "ohms" are not resistance - they are impedances, and represent
the impedance matching between the source (amplifier) and the
atmosphere (the sound production). A mismatch primarily reduces
the efficiency of power transfer. However, because of other factors
in the component behaviors, a mismatch can also affect frequency
response and distortion.

response 10 of 65:

Mar 19 14:54 UTC 2002

That last one is a puzzler; I would have guessed that "Dolby FM" would 
have been a switch position which would also have turned on the 
MPX filter in addition to the Dolby B noise reduction.
 
I still tape "A Prairie Home Companion" and "The Thistle and Shamrock"
(in stereo) on many weekends.

response 12 of 65:

Mar 19 20:28 UTC 2002

I haven't got time to do this justice...  I will have to come back 
and write more later.   

Dolby and dbx Noise Reduction both work by compressing parts of the 
signal on recording and expanding them on playback.  When the signal is
expanded on playback, the volume of the tape hiss, relative 
to the signal, is reduced.   The systems differ in their choice 
of what to compress.

dbx is a straight 2:1 - 1:2 dynamic range compression 
scheme.  This system was very effective 
at nearly elminating tape hiss, but it was so 
aggressive that it sometimes introduced some audible modulation 
of the tape hiss, which was called "pumping" or "breathing."
It was pretty much agreed by everyone that dbx tapes were 
intolerable to listen to on machines which did not have dbx 
decoding.  I never worked with dbx equipment.

In the Dolby techniques, the amount of compression done to 
the signal varies with the audio frequency.  The higher
the frequency -- the more likely the signal is to get lost in tape 
hiss -- the more compression is done.   This is not as effective
as dbx, but it also means that a Dolby-encoded tape can be played
back without decoding, and most people won't object too much to the
resulting sound -- it's a little "bright" because the high frequencies
have been boosted.   This is considerably more complicated than dbx
and to work best, it requires that the tape deck be properly tuned
to the tape being used.

    Dolby A is a professional noise reduction system.  You will not find
    it in home audio equipment.
 
    Dolby B was the first Dolby noise reduction product for home use.
    Any cassette deck you find with a switch marked just "Dolby" 
    is controlling "Dolby B" circuitry.  Dolby B was introduced maybe
    around 1973, for a guess without looking it up. 
    Dolby B gets about 10db of tape hiss reduction -- much less 
    than dbx, but the tapes sound OK if played on equipment without Dolby.

    Dolby C was the second home version, introduced maybe around 1979.
    In a perfectly working system it got 10dB more tape hiss reduction,
    but because it was such an aggressive system it was more audible 
    when there were small tape/machine equalization problems,
    and Dolby C tapes sounded somewhat shrill when played without 
    decoding. 
  
    Dolby S came out after I had stopped paying attention to 
    cassette technology.


Tape selector switches of EX, SX, and ZX tell me you've got your hands
on a Nakamichi brand tape deck, because those were Nakamichi's 
own names for the tape formats.  Some Nakamichi models in good 
working order are quite collectible, selling for hundreds of 
dollars.  Nakamichi was usually regarded as the premier manufacturer
of cassette decks from the early 1970s into the CD era.
However, because Nakamichi and the rest of the world disagreed on 
how equalization was to be done, tapes recorded on Nakamichi machines
sound "dull" on other manufacturer's machines.  I have considerable
experience with this; I lived with or owned three Nakamichi recorders
back in the cassette era.

Tape formulations: roughly equivalent, on each line:
 
 Type I = Normal = Ferric = EX
 Type II = Chrome or Chrome-Equivalent = CrO2 = SX
 Type III = Ferro-Chrome = FeCrO2   <-- this formulation failed in the market
 Type IV = Metal = ZX
           Type IV / Metal tapes can be played back correctly with a 
           switch in the Type II/Chrome position, but they cannot be 
           so recorded.

Actually *explaining* them will have to wait.

response 14 of 65:

Mar 19 21:46 UTC 2002

We have a Kenwood with Bias Preset and Auto Bias that apparently detects the
tape type as that is not a choice. Yes it was Nakamichi with the EX SX ZX.
We have about 20 tape decks, some of which work.  CrO2 is sometimes just
labelled 'high' - high bias?  Some tapes are labelled 120 (I, normal?) and
some 70 (II, IV?) - is this bias or equiv?

We will skip dbx since only one deck has that and take your advice to use
Dolby (B) rather than C.  Don't have any S.  Jim likes to hear things brighter
- perhaps his hearing is shot from too much hammering, but he turns on Dolby
during play for tapes not recorded with it.  

One deck has settings Normal (120 msec), FeCR (70), CrO2 (70) and Metal (70).
Is the FeCr same as FeCrO2 that you mentioned as type III?  Some decks require
you to set both bias and equ and various combinations work for I, II and IV.
For instance both buttons out for I, both in for IV, one in and one out for
II.  A couple of these decks are not understandable.  

If you are interested, Ken, you are welcome to come over and admire and even
play with our tape deck collection.  We are still trying to fix one good one
to the point where it:  plays, records (both channels), and the door works.
We are replacing belts, tires, motors, springs and other bits and pieces.

Were home tape decks first popular around 1973, or even earlier (without
Dolby)?

We have encountered a variety of autoreverse mechanisms.  Usually the head
and tape stay put and the spindles/rollers switch direction, either by some
system of gears, or by acctually reversing motor direction (would this be by
changing polarity to a DC motor?).  One deck swivels the play head, and we
ran across one that actually turned the tape over for you, a very early model.
MCM electronics sells new autoreverse play heads for $2, or the swivelling
type for slightly more (also IEEE printer cables for about $2-3, of the type
that our friend paid $25 for recently).

response 16 of 65:

Mar 20 17:58 UTC 2002

Hoo boy.  I can't explain Dolby HX Pro without covering "bias."
And I'm not even sure I can explain that very well; I may go out
and look for a URL to point you to.
 
In brief:  Dolby HX Pro is not a noise reduction system at all.
"HX" stands for Headroom Extension; it is a circuit to reduce
the bias current a bit when there is lots of high frequency
energy present in the musical signal, to try and stop too much total
high-frequency signal from going into the record heads.  Dolby HX Pro 
is only applied in the recording process; no decoding playback 
is required.

My last high-quality cassette deck had HX Pro, but I eventually 
concluded it didn't help the sound and I turned it off.

response 18 of 65:

Mar 21 17:25 UTC 2002

The marketing for HX Pro argued that when too much high-energy signal
went into the record heads or tape that there was some "self-erasing"
going on, resulting in less-than-optimal high frequency performance.
So that's why it was called "HX Pro," for Headroom Extension.
Once the system accomplished its goal of getting a hotter high-frequency
signal onto the tape, there is no need for any special processing 
to play it back.
 
You know, the more I write this stuff, the more I remember why
I'm recording stuff digitally these days...

response 20 of 65:

Mar 22 23:11 UTC 2002

        I still have an outboard Dolby box.  Control knobs on it 
to control things.  Came with a reel to reel tape and as cassette
tape for calibrations.

response 22 of 65:

Mar 23 23:49 UTC 2002

        I haven't had an eight track recorder for some time.

response 24 of 65:

Mar 24 12:37 UTC 2002

Heh.  Watch out, Tim.